In current SLM projection displays, such as DMD™ or LCD, the projector's lamp emits broadband (white) light, which is split into primary or secondary color components by dichroic surfaces in stationary prisms, mirrors, or rotating panels, such as color wheels or drums. These filtered planes of light are coupled to the projection SLM(s) in synchronization with pixelated image data. In the case of a DMD projector, the projection DMD's mirrors are controlled by the image data to be either in the correct state for color projection to the screen or out of the field of view. Integrators are often added to homogenize the illumination.
DMD based projection displays typically use rotating color wheels, which are optically inefficient, for color-plane separation. These color wheels require motors, balanced parts, and rotary sensors all of which contribute negatively to system reliability, size, safety, noise, maintenance, and cost. The relative duty cycle of each color is constant, determined by the relative dimensions of the filter panels. By design, the filter panels only let specific wavelengths of light through during their working phase, therefore wasting the light of otherwise useful wavelengths.
Other projectors employ stationary color beam splitters that use two or three defect-free SLMs to project distinct color plane images to the screen. In these systems, two or three projection SLMs are optically configured in parallel to pixelate and relay the three-color image planes. Stationary beam splitting prisms have to be precisely aligned (converged) and stabilized to avoid image drift. The projection SLMs require even more precise alignment to prevent out-of-focus images and these are position sensitive to thermal and structural stress.
While multiple projection SLMs in parallel have an advantage in optical efficiency, negative factors are lifetime stability, mis-convergence, maintenance, MTBF, cost, and limited contrast due to light scatter from the constant, full area SLM illumination.
Projectors are configured using one SLM for lower cost systems, two SLMs for higher performance systems, and three SLMs for very high brightness systems. All of these require very low defect projection SLM devices.
An example of a one-DMD projector with a rotating color wheel illumination system is shown in FIG. 1. In this case a light source, consisting of a lamp 100 and a collector 101, directs white light into a first condenser lens 102, which brings the light down to a small focused spot at the surface of a motor 104 driven rotating color filter wheel 103. The light is segmented into sequential red-green-blue primary colors by the color wheel and then resized to fit the DMD by a secondary condenser lens 105, placed in the light path. Sometimes an optional clear segment is added to the color wheel 103 for improved brightness. Light from this secondary condenser lens is focused on to the reflective surface of the DMD 106 where the micro-mirrors, which make up the matrix of pixels, are placed in binary states corresponding to the image data content, by the data path processor 107. Mirrors that are in the ON binary state modulate and reflect sequential color images through a projection lens 108 on to a viewing screen (not shown). The sequential color images projected on the screen are integrated by the observer's eyes to provide a high quality color image. This type projector is lower cost since only one SLM is required and it offers auto-convergence since the color images are exactly laid on top of each other.
FIG. 2a shows a typical configuration for a somewhat higher performance DMD projection display, which used two projection DMD's and a yellow-magenta rotating color wheel. White light from a color source (lamp/collector) 200 is directed into a first condenser lens 202 by a first turning mirror 201. This first condenser lens focuses the light down to a small spot at the surface of a motor 204 driven yellow-magenta color wheel 203. Sequential red-green and red-blue light coming through the color wheel is then resized by a secondary condenser lens 205 and then directed off the surface of a second turning mirror 206 into a total-internal-reflective (TIR) prism 207, which is used to get light into and off of the two DMDs 209/210. Light from the TIR prism 207 is coupled into a color splitting/recombining prism 208, where red light is reflected off an internal surface and focused on to the reflective surface of a first DMD 209 and sequential green-blue light is passed through the prism and focused on to the surface of a second DMD 210. The micro-mirrors of the two DMD's are placed into their binary states based on the image content by a data path processor 211. Red and green-blue light is then modulated by the DMDs and reflected out of the recombining prism 209, back through the TIR prism 207, and through a projection lens 212 on to a viewing screen (not shown).
FIG. 2b shows yet another even higher performance projection display configuration, which also has two projection DMDs and a blue-green rotating color wheel, but in this case the red light is split-off prior to the rotating color wheel. Here, white light from the source 250/collector 251 is directed to a mirror 252, which reflects red light along a first path, through condensing optics (not shown) on to the reflective surface of a first DMD 253, and passes green-blue light into a first condenser lens 254. This first condenser lens focuses the green-blue light down to a small spot at the surface of a motor 256 driven rotating color filter wheel 255. Sequential green and blue light coming through the color wheel is then resized to fit a second DMD 256 by means of a secondary condenser lens 258 and directed on to the surface of the DMD 258. This sequential green-blue light is modulated based on the green-blue image content by data path processor 259 and the red light is modulated based on the red image content by data path processor 260. The sequential green-blue modulated image and constant red modulated image are then combined by combining mirror 261 and the light is directed through a projection lens 262 on to a viewing screen (not shown).
FIG. 3 shows a three-DMD projection display, which provides the highest-performance, highest-brightness of those discussed. In this case there is no rotating color wheel, but the system splits the white light into three constant red, blue, green beams going to respective DMD modulators. Here, light from a white light source 300 is directed through a condenser lens 301 where it is sized and then turned by a turning mirror 302 and coupled into a TIR prism 303. The light is reflected off an internal surface of the TIR prism into a color splitting/recombining prism 304, where it is segmented into three continuous red, green, and blue beams, each being directed on to the reflective surface of a red 305, green 306, and blue 307 DMD. In each case the respective red, green, and blue light is modulated by the DMD micro-mirrors, based on the respective image content, and the respective color image is reflected back into the recombining prism 304. The recombined superimposed red-green-blue images are then directed back through the TIR prism 303, through a projection lens 308 and on to a viewing screen 309. Of course, these images have to be perfectly converged (aligned) in the optics plane to provide a well-focused picture on the screen.
All of these displays have drawbacks resulting from either using the rotating color wheel or from using an uncontrollable white light source. What is needed is a controllable light source that does not require the use of a rotating color wheel. The illumination approach of the present invention meets this need in numerous embodiments by using additional DMD illumination modulators to switch and control the light input to the projection modulators. These illumination modulators can usually have a certain number of defects as compared to the projection modulators, thereby allowing reject devices to be used. Often times the illumination modulators can be smaller in size; i.e., can have fewer pixels. In addition, the illumination modulators can be used to control the amount of light going to certain areas of the projection modulators to lower the dark level and improve the contrast of the projected image, as compared to using constant light levels in current systems. This approach increases the contrast in one-SLM projection systems and improves the optical efficiency in all DMD projection systems.